This is my second digital painting ever. Meaning I'm a total noob at this.
It's not too ba, comparing to what I've seen, but still, very bad and I've got a really long way to go. Through, I'm looking forward to the day when I'll be able to redo this piece and will be actually proud of it.

Please, come up with some suggestions, I'd love to know how to improve this piece and anything I will do in future.

good news, this isn't nearly as bad as you disclaim it. you seem to have a good eye and a good sense of design. i would narrow it down to two things:

1. the light/shadow situation is a bit wishy-washy with no clear direction or definition of form. the colors are too literal (yellow hair is yellow, blue towel is blue, beige skin is beige) - balance out your warm lights with some cool shadows and make the forms really pop.

2. paint holistically. you're really holding tightly to the linework - the razor-sharp edges around the body, the pants, the towel, etc. - making things feel a bit stiff and rendered piecemeal vs. as a unified composition. i suspect you painted the basic silhouette of each element and dodge/burned them individually, which makes the work feel cobbled-together. and i think that's largely a matter of experience and confidence - grab the paintbrush and just paint directly. use those hard edges sparingly to draw focus where necessary, and let there be some softer, less defined areas as the lighting would dictate. a quick search came up with this example: http://static2.artspan.com/member/fimam/500/105365.jpg

the shoulders and the hand almost bleed into their surroundings with no edges at all, making the definition in her back speak volumes. it's about giving the eye a specific place to land, allowing certain areas to lead the composition and other areas to simply suggest themselves. otherwise, every part of the painting competes for attention. my eye is bouncing from the edges of his torso to the necklace to the waistline of his pants to the edges of the towel, and round and round - i'm looking at the painting, but i'm not drawn in. particularly the edges of the torso - if there was ever a time for soft edges, it's here.

i have no formal training so i struggle to articulate these things, but essentially i would advise the following: don't be afraid to paint outside of the lines, create your forms and edges deliberately and in harmony with the composition of the whole. define your focal point(s), and let those areas lead the eye while the less important areas remain a little more vague - the viewers can fill in the blanks for themselves. and remember the interplay of warms/cools in your palette - you have a very warm lighting scheme here, push it further with some cool shadows. there was an incredible illustrator named bengal who posted here back in the day, who understood that dynamic and used it to maximum impact. here's an example: http://farm5.staticflickr.com/4002/4421034084_fc6f03acd0_o.jpg

those blues and purples in the shadows complement the warmer highlights, and create a sense of dimension and form.

finally, don't get caught up in details until you've established your basic framework and have your values in place. force yourself to use only large, soft brushes until you have a palpable foundation in place - then begin the rendering process. a wise man once said that your painting should be able to stand on its own at any given point - whether you've been at it for 10 minutes or 10 hours, a solid foundation is a solid foundation regardless of how much noodling detail you pile on top of it. you should be able to call your painting finished at any moment. but you have to start with a proper foundation of values. try converting this to grayscale, you'll see it lives largely in midtones.